This invention relates to a solid state relay circuit, and more specifically relates to a solid state relay circuit employing a power MOSFET as the main switching device and having extremely rapid turn-on and turn-off capability when driven from the output of a single input voltage source.
Solid state relay circuits are well known wherein a power MOSFET or other switching transistor is used as the main power switching device. Such relays are also known wherein the input energy for switching the transistor is derived from a photovoltaic generator which can be illuminated by a suitable LED or other radiation source in order to produce an output current which causes the switching of the device. Such a relay is shown in U.S. Pat. No. 4,227,098.
When the main power switching device is a power MOSFET, the input current from the photovoltaic source must charge the gate capacitance of the device sufficiently in order to turn the device on. When using photovoltaic generators, commonly a stack of solar cell-type devices, such generators must operate into a high impedance to prevent the diversion of the cell output current away from the MOSFET gate capacitance. The need for a high impedance delays the discharge of the gate capacitance when the input radiation signal to the photo-generator is turned off and the photogenerator output voltage collapses. Thus, in the circuit of Patent 4,227,098, the main power MOSFET remains on after the input signal terminates for the length of time necessary to discharge the gate capacitance of the power MOSFET into a high impedance circuit. The circuit of Patent 4,227,098 is also sensitive to false firing due to high dV/dt across the power MOSFET terminals since the high dV/dt will charge the drain-to-gate capacitance of the power MOSFET and turn on the relay without an input signal.
Circuits are known to cause more rapid discharge of this gate capacitance for high speed turn-off. Such circuits, however, employ a second photovoltaic source, as is shown in U.S. Pat. No. 4,390,790. The second photovoltaic source or array is used to sense the presence or absence of an input signal and turns on a depletion mode MOSFET when the input illumination is turned off with the turn-off of the input signal. The MOSFET gate capacitance can then discharge more quickly through the conducting depletion mode MOSFET for higher relay turn-off speed.